The invention relates to systems and methods for optical probing of an electrical device such as an integrated circuit.
Electrical devices, such as integrated circuits (ICs), may fail for a variety of reasons. Particular defects within an electrical device may be identified by optical probing of the device as the device is being conditioned by test equipment capable of exercising the device under normal clock operating rates. ATE (automated test equipment) production testers combine device level biasing with sophisticated software for delivering test signals (vectors) to the device at normal operating rates. A variety of optical techniques have been described for evaluating the performance of an electrical device under test (DUT). For example, optical techniques which detect charge density variations in an electrical DUT may be used to monitor logic states and signal levels at internal nodes within the electrical device. These techniques are particularly useful for monitoring and diagnosing the performance of circuit nodes that are not readily accessible to non-optical probes. For example, the device side of many ICs (so-called flip-chip devices) are inaccessible to probes. Thus, testing and debugging of such devices must be done from the back side, using techniques described, for example, in K. R. Wilsher and W.K. Lo, Practical Optical Waveform Probing of Flip-Chip CMOS Devices, Proceedings, ITC International Test Conference, paper 35.1, pp. 932-39 (hereafter, xe2x80x9cWilsher and Loxe2x80x9d), which is incorporated herein by reference.
In general, in one aspect, the invention provides an optical probe system for optically probing an electrical device while the device is operating under control of a tester of the kind that generates and applies a sequence of test vectors to the device and that has a test head in which the device can be mounted. The optical probe system has a light delivery and imaging module that is configured to be docked to a test head and that has imaging optics and a fine scanner. The optical probe system also has an optical processing subsystem that can generate an incoming beam of light to illuminate the device; a data processing subsystem that can generate control signals for the module and that can transmit control signals to the module; and an optical fiber connected to transmit the incoming beam from the optical processing subsystem to a fiber end in the module. The fine scanner has a frame and a platform and can receive control signals from the data processing subsystem to move the platform relative to the frame. The fiber end is mounted in a fixed position on the optical axis of the imaging optics so that light emitted from the fiber end is focused at a focal point in a focal plane by the imaging optics, and the fiber end and imaging optics are mounted in a fixed position to the platform of the fine scanner, whereby operating the fine scanner moves the fiber end, the imaging optics, the optical axis, and the focal point as a rigid unit.
Advantageous implementations can includes one or more of the following features. The system further includes a coarse stage having a frame and a platform, the platform being connected to the fine scanner in a fixed relationship, the coarse stage being coupled to receive control signals from the data processing subsystem to move the platform of the coarse stage. The fine scanner can move the imaging optics and the fiber end in an XY plane perpendicular to the optical axis and in a Z direction parallel to the optical axis; and the fine scanner is so positioned in the module that the focal point moves in a focal plane intersecting the device when the fine scanner moves the fiber end in the XY plane and the module is docked to the test head. The fiber is a polarization-maintaining, single-mode fiber; the incoming beam of light is monochromatic and linearly polarized; and the fiber end is polished so that the beam is spatially filtered. The focal point and the fiber end are conjugate focal points of the imaging optics; and the fiber end defines an aperture that delivers light to and from the imaging optics, whereby light delivered to the imaging optics through the aperture and light reflecting from the focal plane passes through the aperture. The optical processing subsystem includes a polarizing beam splitter configured to pass light from a light source with a first polarization and to reflect light with a different polarization toward a detector, and it is configured to deliver the light from the beam splitter to the fiber; and the imaging optics have a quarter-wave plate on the optical axis between the fiber end and the focal point. The fiber is a single-mode fiber, and the incoming beam of light is monochromatic. The incoming beam of light is produced by a pulsed laser and has a near infrared wavelength.
The fiber is a polarization-maintaining, single-mode fiber; the incoming beam of light is monochromatic and linearly polarized; the fiber end is polished so that the beam is spatially filtered; the system further comprises a coarse stage having a frame and a platform, the platform being connected to the fine scanner in a fixed relationship, the course stage being coupled to receive control signals from the data processing subsystem to move the platform of the coarse stage; the focal point and the fiber end are conjugate focal points of the imaging optics; the fiber end defines an aperture that delivers light to and from the imaging optics, whereby light delivered to the imaging optics through the aperture and light reflecting from the focal plane passes through the aperture; the optical processing subsystem comprises a polarizing beam splitter configured to pass light from a light source with a first polarization and to reflect light with a different polarization toward a detector, the optical processing subsystem being configured to deliver the light from the beam splitter to the fiber; and the imaging optics comprise a quarter-wave plate on the optical axis between the fiber end and the focal point. The optical processing subsystem and the data processing subsystem are housed in a common probe system chassis.
In general, in another aspect, the invention provides a method for optically probing an electrical device. The method includes operating the device; providing imaging optics, the imaging optics having an optical axis and a focal point on the optical axis, the imaging optics positioned in a spatial relationship to the device so that the focal point intersects the device; providing an aperture on the optical axis through which light is delivered to the imaging optics; moving the aperture, the optical axis, and the imaging optics as a rigid unit to move the focal point to a reference point in the device; and providing a beam of light to the aperture and receiving reflected light from the device.
Advantageous implementations can includes one or more of the following features. The device is operated under a repeating sequence of test vectors. The method further includes raster scanning the focal point over an area of the device while illuminating the device with light delivered through the aperture. The method further includes placing the aperture at a conjugate to the focal point and receiving the reflected light through the aperture.
In general, in another aspect, the invention provides an optical probe system for optically probing an electrical device while operating the device under control of a tester. The optical probe system has a light delivery and imaging module. The module has imaging optics, a coarse stage and a fine scanner, and is configured to be docked to a test head. The system has an optical processing subsystem operable to generate an incoming beam of light to illuminate the device; a data processing subsystem operable to generate control signals for the module and operatively coupled to transmit control signals to the module, the coarse stage and the fine scanner being coupled to receive control signals from the data processing subsystem; and an optical fiber connected to transmit the incoming beam from the optical processing subsystem to a fiber end in the module. The imaging optics have an optical axis, a focal plane, and a conjugate focal plane. A frame of the fine scanner and the imaging optics are mounted in a fixed position relative to a platform of the coarse stage, whereby operating the coarse stage moves the imaging optics, the optical axis, and the frame of the fine scanner as a unit. The fiber end is mounted on a platform of the fine scanner so that operating the fine scanner moves the fiber end in the conjugate plane and so that light emitted from the fiber end is focused in the focal plane by the imaging optics, whereby operating the coarse stage moves the fiber end, the imaging optics, the optical axis, and the focal point as a unit, and operating the fine scanner moves the fiber end and focal point relative to the imaging optics.
Advantageous implementations can includes one or more of the following features. The fine scanner can move the fiber end in an XY plane perpendicular to the optical axis and in a Z direction parallel to the optical axis; and the fine scanner is so positioned in the module that the focal point moves in a focal plane intersecting the device when the fine scanner moves the fiber end in the XY plane and the module is docked to the test head. The fiber end defines an aperture that delivers light to and from the imaging optics, whereby light delivered to the imaging optics through the aperture and light reflecting from the focal plane passes through the aperture. The optical processing subsystem includes a polarizing beam splitter configured to pass light from a light source with a first polarization and to reflect light with a different polarization toward a detector, where the optical processing subsystem is configured to deliver the light from the beam splitter to the fiber; and the imaging optics include a quarter-wave plate on the optical axis between the fiber end and the focal point.
In general, in another aspect, the invention provides a method for optically probing an electrical device. The method includes operating the device; providing imaging optics, the imaging optics having an optical axis and a focal plane, the imaging optics being positioned in a spatial relationship to the device so that the focal plane intersects the device; providing an aperture through which light is delivered to the imaging optics, positioned so that light from the aperture is focused at a focal point; moving the imaging optics in a plane parallel to the focal plane; moving the aperture relative to the imaging optics to move the focal point to a reference point in the device; and providing a beam of light to the aperture and receiving reflected light from the device.
Among the advantages that can be seen in particular implementations of the invention are the following. The beam delivery and imaging subsystem requires only a small number of components and may be of small size and low weight and can therefore be mounted quickly and easily to the test head of an ATE system in contrast to prior art systems that require the imaging module to be docked to the test head of the ATE system. The invention can achieve good imaging results because the beam delivery and imaging subsystem can be substantially decoupled from sources of vibrational noise (e.g., vacuum pumps) and can be rigidly coupled to the ATE test head. The small size of the beam delivery and imaging subsystem provides a short mechanical path (allowing little relative vibration) between the scanner optics and the DUT. The beam delivery and imaging subsystem can be made from a small number of optical elements, which can contribute to good transmission efficiency. The system can perform optical probing during parametric and functional testing of a delidded or flip-chip device mounted in an ATE tester test head.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the description, the drawings, and the claims.